The present invention relates to fastener-driving tools, also known as power nailers, which are typically powered by combustion, pneumatics, electricity, are powder-activated or otherwise powered. In such tools, a plurality of fasteners are sequentially arranged in a magazine and are urged by a biased follower toward a driving end of the magazine where the fasteners are each pushed into a nosepiece. Once in the nosepiece, the fasteners are driven into a workpiece by a reciprocating driver blade.
A design criterion of most such tools is that the tool should be disabled when the magazine is empty of fasteners. So-called “dry firing” or “blank firing” generates significant stresses in the tool and can damage the workpiece or the tool itself. Indicator mechanisms are known in such tools to indicate to the user when the magazine is empty or almost empty, so that the magazine can be refilled prior to a dry firing condition. In some of these known mechanisms, once a magazine follower reaches a preset point where a relatively few fasteners remain in the magazine, the tool is disabled by locking a workpiece contact element to prevent dry firing.
While the prevention of dry firing is common to many types of fastener-driving tools, it is particularly challenging when the fasteners are finish nails, which typically are provided in stamped strips. The problems associated with driving such fasteners are described in commonly assigned U.S. Pat. No. 6,176,412 which is incorporated by reference. Each fastener is relatively thin, and this reduced thickness results in a relatively small increment of movement of the follower upon the driving of each fastener. Thus, it is difficult to design a tool using such fasteners to effectively alert the user when the magazine needs refilling.
A related design problem is that visual indicator systems for preventing dry firing require attention by the operator for effectiveness. If the user is understandably preoccupied with driving fasteners, the status of the magazine may be overlooked, resulting in a dry firing situation due to an empty magazine. In some cases, audible warning systems may also be overlooked when the user is concentrating on fastener application.
A related design issue is that in some cases, leftover fastener strip segments are prone to “tumble” or become misaligned within the magazine. Such segments can cause the tool to jam, especially in applications where the nail follower or pusher in the magazine fails to strongly urge the fasteners toward the nosepiece. This problem is especially severe when plastic collated fasteners are used. Since the plastic collation media is relatively brittle, the tendency is for the fasteners to become detached from the media and tumble around inside the magazine.
Thus, there is a need for providing a power nail feeding mechanism for a fastener-driving tool which prevents the tumbling or misalignment of fastener strip segments within the magazine. There is also a need for providing a power nail feeding mechanism for such a tool which prevents jams in either the magazine or the nosepiece due to stray or misaligned fastener strip segments. There is also a need for a more effective system for preventing dry firing in fastener-driving tools.